The invention relates to vitrified bonded abrasive tools made with a high strength, low temperature bond, comprising phosphorous oxide and boron oxide in amounts sufficient to improve the performance of an abrasive tool containing sintered sol gel alumina abrasive grain. As a result of the bond selection, sintered sol gel alumina abrasive grain (or other thermally labile abrasive grain) may be used effectively, without loss of grinding performance in the abrasive tool.
The invention further includes a vitrified bond composition suitable for firing at relatively low temperatures such as 700-1,100xc2x0 C., comprising at least two amorphous, immiscible, glass phases. Abrasive tools comprising superabrasives (diamond or cubic boron nitride (CBN)), or seeded or unseeded sintered sol gel alumina abrasive grain, also referred to microcrystalline alpha-alumina (MCA) abrasive grain, are known to provide superior grinding performance on a variety of materials. The manufacture and characteristics of these MCA grains and the performance of these MCA grains in various applications are described in, for example, U.S. Pat. Nos. 4,623,364, 4,314,827, 4,744,802, 4,898,597 and 4,543,107, the contents of which are hereby incorporated by reference.
Vitrified or glass bonded abrasive tools containing MCA grain and superabrasive grain are commercially useful for grinding precision metal parts and other industrial components requiring consistent and improved grinding performance. To produce these types of abrasive tools with consistent quality, reactions between glass bond components and the abrasive grain must be avoided. Reactivity is a particular problem at typical temperatures encountered during firing of the bond, e.g., 1100-1400xc2x0 C. Controlling these reactions minimizes damage to the critical microcrystalline structure of the MCA grain, preserving the grain sharpness and performance.
To reduce the amount of reaction between MCA grain and vitrified bond, U.S. Pat. No. 4,543,107 discloses a bond composition suitable for firing at a temperature as low as about 900xc2x0 C. In an alternate approach, U.S. Pat. No. 4,898,597 discloses a bond composition comprising at least 40% fritted materials suitable for firing at a temperature as low as about 900xc2x0 C. However, in certain grinding applications these low temperature bonds have demonstrated insufficient mechanical strength to meet commercial objectives prompting development of stronger bonds.
Vitrified bonds characterized by improved mechanical strength have been disclosed for use with either conventional fused alumina oxide or MCA (also referred to as sintered sol gel alpha-alumina) abrasive grits in manufacturing grinding wheels having improved form holding properties. Such bonds are described in U.S. Pat. No. 5,203,886, U.S. Pat. No. 5,401,284 and U.S. Pat. No. 5,536,283, which are hereby incorporated by reference. These vitrified bonds may be fired at relatively low temperatures (e.g., about 900-1100xc2x0 C.) to avoid reaction with high performance, sintered sol gel alpha-alumina abrasive grain. The wheels made with these bonds and MCA grain have shown excellent performance in finishing precision moving parts, particularly ferrous metal parts. Other vitrified bonds suitable for use with MCA abrasive grain may be fired at temperatures below about 875xc2x0 C. These bonds are disclosed in U.S. Pat. No. 5,863,308, which is hereby incorporated by reference.
It has now been discovered that by selecting appropriate material components, improved high strength, tough bonds may be made and fired at about 700 to 1,100xc2x0 C., preferably 750 to 950xc2x0 C. In particular, by selecting appropriate contents of phosphorous oxide, boron oxide, silica, aluminum oxide, alkali oxides and alkaline earth oxides, and by maintaining the correct ratios of oxides, one may achieve a high strength, tough (e.g., resistant to crack propagation), low temperature bond. These bonds are characterized by a 25% or larger increase in modulus of rupture value relative to comparative bonds of the prior art. In certain embodiments, bonds comprising at least two amorphous, immiscible, glass phases may be used with MCA grain to yield greater mechanical strength. While the appropriate selection of raw materials having the desired oxide ratios upon firing can achieve a glass with immiscible phases, fritted glasses are preferred for this purpose. A fritted glass is a glass formed by firing initially to temperatures of at least 1,200xc2x0 C., cooling, crushing and sizing to yield a powdered material (xe2x80x9ca fritxe2x80x9d). The frit then may be melted at a temperature well below the initial firing temperature used to make the glass from the raw materials, such as silica and clays.
When formulating an abrasive tool, such as an abrasive wheel or hone, the use of these vitrified bonds with superabrasive or MCA grain, yields abrasive tools having improved grinding performance with reduced power draw. When used to grind or finish a workpiece, these abrasive tools yield very acceptable workpiece surface finishes. These tools offer improvements over the low temperature fired, vitrified bonded superabrasive or MCA grain tools previously known in the art.
The invention is an abrasive tool comprising at least 1%, by volume, MCA abrasive grain and 3 to 30%, by volume, vitrified bond, wherein the vitrified bond comprises after firing of the abrasive tool, 40 to 60% SiO2, 10 to 18% Al2O3, 12 to 25% alkali oxides, 5 to 20% B2O3, and 1 to 8% P2O5, on a mole percent basis, and whereby the abrasive tool is characterized by at least a 30% increase in modulus of rupture relative to an comparable abrasive tool made with a vitrified bond comprising less than 1 mole % P2O5 The commonly used hardness grades of abrasive tools containing MCA grain (e.g., K grade and harder on the Norton Company scale) are characterized by having a modulus of rupture of at least 6,000 psi when made according to the invention.
The alkali oxides of the bond are selected from the group consisting of sodium oxide, lithium oxide and potassium oxide.
The abrasive tool preferably comprises 5 to 25 volume % vitrified bond and 10 to 56 volume % MCA abrasive grain, and may comprise about 0.1 to about 60 volume % of additional components selected from the group consisting of secondary abrasive grains, fillers and adjuncts. The vitrified bond after firing may comprise alkaline earth oxides, and the molar ratio of SiO2 to the combined contents of Na2O, alkali oxides other than Na2O and alkaline earth oxides is at least 1.2:1.0.
The invention further is an abrasive tool comprising at least 1%, by volume, MCA abrasive grain and 3 to 30%, by volume, of a vitrified bond, wherein the vitrified bond, during firing of the abrasive tool at about 700 to 1,100xc2x0 C., comprises at least two immiscible phases, and whereby the abrasive tool is characterized by at least a 30% increase in modulus of rupture relative to an comparable abrasive tool having a single phase vitrified bond.
The vitrified bond having at least two immiscible phases preferably comprises a maximum of 12 mole % Al2O3.
Either bond may further comprise fluorine, TiO2, ZnO, ZrO2, CaO, MgO, CoO, MnO2, BaO, Bi2O3, and Fe2O3, and combinations thereof.
The invention also includes a method for making an abrasive tool comprising the steps of:
a) mixing about 70 to 95 weight % abrasive grain selected from the group consisting of MCA grain, silicon carbide grain, diamond grain, and cubic boron nitride grain, and mixtures thereof, and about 5 to 30 weight % bond mixture, wherein the vitrified bond comprises after firing of the abrasive tool, 40 to 60% SiO2, 10 to 18% Al2O3, 12 to 25% alkali oxides, 5 to 20% B2O3, and 1 to 8% P2O5, on a mole percent basis;
b) molding the mixture into a green composite; and
c) firing the green composite at a temperature in the range of 700 to 1,100xc2x0 C. to form the abrasive tool; and whereby the abrasive tool is characterized by at least a 30% increase in modulus of rupture relative to an comparable abrasive tool made with a vitrified bond comprising less than 1 mole % P2O5.
The method is particularly useful for abrasive grain selected from the group consisting of MCA grain, silicon carbide (SiC) grain, diamond grain, cubic boron nitride grain, and mixtures thereof. The firing step of this method may be carried out in an oxidizing atmosphere.
The invention further includes a microabrasive finishing tool, such as a hone or a stone, and grinding wheels made with MCA grain and having improved grinding performance, particularly in yielding a smooth surface finish on precision moving parts.
The vitrified bonded abrasive tools of the present invention comprise MCA abrasive grain. The MCA or sol-gel alumina grain are preferably produced by either a seeded or an unseeded sol gel process. As used herein, the term xe2x80x9csol-gel alumina gritsxe2x80x9d are alumina grits made by a process comprising peptizing a sol of an aluminum oxide monohydrate so as to form a gel, drying and firing the gel to sinter it, and then breaking, screening and sizing the sintered gel to form polycrystalline grains made of alpha alumina microcrystals (e.g., at least about 95% alumina).
In addition to the alpha alumina microcrystals, the initial sol may further include up to 15% by weight of spinel, mullite, manganese dioxide, titania, magnesia, rare earth metal oxides, zirconia powder or a zirconia precursor (which can be added in larger amounts, e.g. 40 wt % or more), or other compatible additives or precursors thereof. These additives are
often included to modify such properties as fracture toughness, hardness, friability, fracture mechanics, or drying behavior.
Many modifications of alpha alumina sol gel grain have been reported. All grains within this class are suitable for use herein and the term MCA grain is defined to include any grain comprising at least 60% alpha alumina microcrystals having at least 95% theoretical density and a Vickers hardness (500 grams) of at least 18 GPa at 500 grams. The microcrystals typically may range in size from about 0.2 up to about 1.0 microns for seeded grain and from greater than 0.2 to about 5.0 microns for unseeded grain. The sintered sol gel alpha-alumina grain may contain platlets of material other than alpha-alumina dispersed among the alpha-alumina microcrystals. Generally, the alpha-alumina particles and the platlets are submicron in size when made in this form.
The preparation of sintered sol gel alpha-alumina grains is described in detail elsewhere. Details of such preparations may be found, for example, in U.S. Pat. Nos. 4,623,364, 4,314,827, and 5,863,308, the contents of which are hereby incorporated by reference. Further details of MCA abrasive grain preparations and MCA abrasive grain types useful in the present invention may be found in any one of the numerous other patents and publications which cite the basic technology disclosed in the U.S. Pat. No. 4,623,364 and U.S. Pat. No. 4,314,827.
The abrasive tools of the invention comprise at least 1 volume % MCA abrasive grain and 3 to 30 volume % vitrified bond. The tools typically include 35 to 65 volume % porosity and, optionally, 0.1 to 60 volume % of one or more secondary abrasive grains, fillers and/or additives. The abrasive tools preferably comprise 3 to 56 volume % MCA abrasive grain. The amount of abrasive grain used in the tool and percentage of secondary abrasive may vary widely. The compositions of the abrasive tools of the invention preferably contain a total abrasive grain content from about 34 to about 56 volume %, more preferably from about 40 to about 54 volume %, and most preferably from about 44 to about 52 volume % grain.
The MCA abrasive preferably provides from about 1 to about 100 volume % of the total abrasive grain of the tool and more preferably from about 10 to 80 volume %, and most preferably, 30 to about 70 volume % of the total volume % abrasive grain in the tool.
When secondary abrasive grains are used, such abrasive grains preferably provide from about 0.1 to about 97 volume % of the total abrasive grain of the tool, and more preferably, from about 30 to about 70 volume %. The secondary abrasive grains which may be used include, but are not limited to, alumina oxide, silicon carbide, cubic boron nitride, diamond, flint and garnet grains, and combinations thereof.
The compositions of the abrasive tools optionally contain porosity. The compositions of the abrasive tools of the invention preferably contain from about 0.1 to about 68 volume % porosity, more preferably contain from about 28 to about 56 volume %, and most preferably contains from about 30 to about 53 volume %. The porosity is formed by both the natural spacing provided by the natural packing density of the materials and by conventional pore inducing media, including, but not limited to, hollow glass beads, ground walnut shells, beads of plastic material or organic compounds, foamed glass particles and bubble alumina, elongated grains, fibers and combinations thereof.
The abrasive tools of the present invention are bonded with a vitrified bond. The vitrified bond used contributes significantly to the improved grinding performance of the abrasive tools of the present invention.
The composition of the abrasive wheel preferably contains from about 3 to about 25 volume % bond, more preferably contains from about 4 to about 20 volume % bond, and most preferably contains from about 5 to about 18.5 volume % bond.
The raw materials for the bond may include clay, Kaolin, sodium silicate, alumina, lithium carbonate, borax pentahydrate, borax decahydrate or boric acid, and soda ash, flint, wollastonite, feldspar, sodium phosphate, calcium phosphate, and various other materials which have been used in the manufacture of vitrified bonds. Frits are preferably used in combination with the raw materials or in lieu of the raw materials. These bond materials in combination preferably contain the following oxides: SiO2, Al2O3, Na2O, P2O5 Li2O, K2O and B2O3. Alkaline earth oxides, such as CaO, MgO and BaO, are frequently present, along with ZnO, ZrO, F, CoO, MnO2, TiO2, and Bi2O3.
P2O5 and B2O3 Containing Bonds
The bond after firing contains less than about 55 mole % SiO2, preferably from about 40 to about 50 mole % SiO2; less than about 12 mole % Al2O3, preferably from about 6 to about 11 mole % Al2O3; greater than about 2.5 mole % Li2O, preferably from about 3.5 to about 8.0 mole % Li2O; greater than about 8 mole % B2O3, preferably from about 10 to about 25 mole % B2O3, and about 1 to 8 mole % P2O5, preferably 2 to 6 mole % P2O5. In most bonds of the invention, alkali oxides include, on a bond mole % basis, from about 4 to about 16 mole % Na2O, and more preferably from about 5 to about 10 mole % Na2O; and about 2.5 to 6.0 mole %, of K2O. Cobalt oxide (CoO) and other color sources are not necessary for the invention but may be included where bond color is desirable. Other oxides, such as Fe2O3 and TiO2, and alkaline earth oxides including CaO, MgO and BaO, exist as impurities in the raw materials and may be present in or added to the bond of the invention.
Alkaline earth oxides may be used in the bond of the invention such that the vitrified bond after firing comprises a molar ratio of SiO2 to the combined contents of alkaline earth oxides and alkali oxides of at least 1.2:1.0, when the bond comprises a maximum of 60 mole % SiO2. Greater amounts of these combined oxides, relative to the SiO2, may cause the bond of the invention to be too soft for many grinding operations.
Phosphorous oxide, in combination with boron oxide and controlled ratios of alkali oxides, has application in bonds which have proven to be particularly useful in making vitrified microabrasive stones and hones from MCA abrasive grain for precision finishing operations.
In a preferred embodiment, abrasive tools for superfinishing comprise MCA abrasive grain in a microabrasive grit size, and a vitrified bond comprising 40-55 wt % (46-59 mole %) of SiO2, 15-25 wt % (10-18 mole %) of Al2O3, 11-21 wt % (12-25 mole %) in total of monovalent alkali metal oxide (R2O) and divalent alkaline earth metal oxide (RO), 5-15 wt % (5-15 mole %) of B2O3 and 3-15 wt % (1-8 mole %) of P2O5, in amounts selected to yield a total of 100 wt % (or mole %).
These P2O5-containing vitrified bonds offer the following advantages. Because P2O5 serves to aid melting of the vitrified bond, it becomes possible to fire the superfinishing tool at a relatively low temperature of, e.g., 900xc2x0 C. to 1,000xc2x0 C., to avoid adverse effect on the grinding performance of MCA grain. Other components that aid melting of vitrified bonds include B2O3 and monovalent alkali metal oxides (R2O), but these components tend to drastically lower the melt viscosity of the bond, and, therefore, present a problem in terms of stability of the vitrified bond during manufacture of the abrasive tool. These components can promote chemical reaction between the vitrified bond and the MCA abrasive grain, which may prevent the properties of the fine crystalline structure of the MCA grain from being expressed. In contrast, P2O5 causes little change in the melt viscosity of the bond, and allows the properties of the fine crystalline structure of the MCA grain to be expressed. While divalent alkaline earth metal oxides (RO) have the same action, it is not as notable as P2O5, B2O3 and monovalent alkali metal oxides (R2O). The P2O5 component has good chemical affinity for Al2O3 components such as aluminum phosphate compounds.
The thermal expansion coefficient of the vitrified bond is preferably matched as closely as possible with the abrasive grits. Generally, when the difference between the thermal expansion coefficients of the abrasive grits and the vitrified bond is xc2x12xc3x9710xe2x88x926 or greater, cracks occur in the bond and promote premature shedding of the abrasive grain. The thermal expansion coefficient for alumina abrasive grain is about 8.0xc3x9710xe2x88x926. The B2O3 component acts to lower the thermal expansion coefficient, and it is mainly used to aid melting of vitrified bonds employing superabrasive grains with a low thermal expansion coefficient. Monovalent alkali metal oxides (R2O) act to increase the thermal expansion coefficient. Consequently, when B2O3 or a monovalent alkali metal oxide (R2O) is added to aid in melting of the vitrified bond, the relative amount can prevent the thermal expansion coefficient from matching that of the abrasive grains, resulting in cracking of the bond and accelerated shedding of the abrasive grains. In contrast, while P2O5 has the effect of increasing the thermal expansion coefficient, the increase is not as great as with a monovalent alkali metal oxide (R2O).
Addition of P2O5 to the vitrified bond allows firing to be accomplished at a temperature of 700xc2x0 C. to 1,100xc2x0 C., preferably 850xc2x0 C. to 1,050xc2x0 C., most preferably 900xc2x0 C. to 1,000xc2x0 C. while it undergoes effective chemical bonding with the fine crystalline sintered alumina abrasive grits and can closely match the thermal expansion coefficient of the abrasive grits to prevent premature loss of the abrasive grains from the tool during grinding, thus making possible a superfinishing abrasive tool with satisfactory grinding properties and a long usable life as a result of the enhanced sharpness and grinding action provide by the fine crystalline sintered alumina abrasive grits. Particularly superior performance may be obtained by including P2O5 at 3-15 wt % (1-8 mole %) of the bond. The P2O5 component exhibits peak performance at 6-12 wt % (2.5-6.5 mole %).
When SiO2 content is less than 40 wt % the strength of the bond is reduced, and when it is greater than 55 wt % the melt temperature increases, so that a higher firing temperature is required. When the Al2O3 content is less than 15 wt % a problem results in terms of bond stability, and at greater than 25 wt % the melting temperature of the bond increases, so that a higher firing temperature is required. When the R2O (where R is an alkali metal)+RO (where R is an alkaline earth metal) content is less than 11 wt %, the melting temperature of the bond increases so that a higher firing temperature is required, and when it is greater than 21 wt % a problem results in terms of bond stability. When the B2O3 content is less than 5 wt %, the melting temperature of the bond increases so that a higher firing temperature is required, and when it is greater than 15 wt % a problem results in terms of bond stability.
Immiscible Phase Bonds
The phase separated glass bonds of the invention may be prepared from any glass composition susceptible to phase separation under the conditions useful for manufacture of abrasive tools comprising MCA abrasive grain or other thermally and/or chemically labile grain. Phase separation occurs when a single phase glass separates into two glass phases, each having a distinct chemical composition and material properties. When glass is in a liquid state, the liquid phases are immiscible.
In the case of abrasive tools, amorphous phase separation into immiscible phases allows one to obtain a high strength, tough, glass bond at a relatively low processing temperature. By careful control of oxide ratios in the glass and selection of frits, the majority, or matrix phase of the glass bond will be matured into a higher temperature glass, capable of imparting strength and toughness to the abrasive tool. The minority, or discontinuous phase will be matured into a lower temperature glass, capable of flowing, wetting and bonding the abrasive grains in the relatively low temperature range of 700 to 1,100xc2x0 C.
A preferred embodiment of the phase separated vitrified bond combines phosphates, e.g., phosphorous oxide derived from sodium or calcium phosphate raw materials, to lower the firing temperature with silicate components to lend mechanical strength. The silicate components are preferably provided as an alkaliborosilicate glass system, e.g., Na2Oxe2x80x94B2O3xe2x80x94Al2O3xe2x80x94SiO2, or Na2Oxe2x80x94B2O3xe2x80x94SiO2. The amount of aluminum oxide must be controlled as an excess (e.g., more than 12 mole % in a typical vitrified bond system) will tend to prevent separation into immiscible phases. Higher amounts of boron relative to alkali improve phase separation. Ratios of boron oxide to alkali oxide of 5.25:1 to 1:1 are preferred, and the precise ratio depends upon the amount of aluminum oxide present and whether other modifiers are being used. The addition of other modifiers, such as CaO, MgO, alkali oxides and fluorides, in amounts up to about 2 mole %, typically will enhance separation, particularly when used with phosphorous materials. Oxides such as lithium oxide may be substituted for sodium oxide, particularly when a frit is used.
Without being bound by any particular theory, it is believed the phase-separated bonds are an improvement over similar, non-phase separated bonds for several reasons. The immiscible phase glass bonds yield stronger bond behavior at lower processing temperatures, or they yield increased abrasive tool strength after processing at conventional bond firing temperatures. An increased abrasive tool toughness is due to crack tip blunting or deflection as the crack propagates through the residual stress fields resulting from a multi-phase solid system. Control is improved over the reaction between abrasive grain and bond as a result of the lowered firing temperatures and the separation of more grain reactive components (e.g., alkali oxides) into the desired phase.
Enhanced grinding performance can be expected as a result of certain lower temperature glass phases undergoing glass transition during grinding. This should increase the effective heat capacity of the abrasive tool, thereby acting to remove heat of grinding from the surrounding abrasive grain and workpiece.
In manufacturing the abrasive tools containing these bonds, organic binders are preferably added to powdered bond components, fritted or raw, as molding or processing aids. These binders may include dextrins and other types of glue, a liquid component, such as water or ethylene glycol, viscosity or pH modifiers and mixing aids. Use of binders improves wheel uniformity and the structural quality of the pre-fired or green pressed wheel and the fired wheel. Because the binders are burned out during firing, they do not become part of the finished bond or abrasive tool.
The abrasive wheels may be fired at the relatively low temperatures indicated herein by methods known to those skilled in the art. The firing conditions are primarily determined by the actual bond and abrasives used. The bond is fired at 700 to 1,100xc2x0 C., preferably 750 to 950xc2x0 C., to provide the mechanical properties necessary for grinding metals and other workpieces. The vitrified bonded body further may also be impregnated after firing in a conventional manner with a grinding aid, such as sulfur or wax, or with a vehicle, such as epoxy resin, to carry a grinding aid into the pores of the wheel.
For microabrasive grain used in abrasive tools for, e.g., superfinishing operatoins, the particle size of the abrasive grits used is typically about 2 to 6 microns (280-6000 JIS grade), but a fine particle size of 18 microns (1000 JIS) or smaller is more common. The bond hardness of the abrasive tool is from +100 to xe2x88x9260 in terms of Rockwell hardness, with +100 being harder and xe2x88x9260 being softer.
The dimensions and shape of the superfinishing tool are generally determined by the workpiece and the mechanical structure. Minimum dimensions are about 2xc3x972xc3x9715 mm and maximum dimensions about 25xc3x9750xc3x97120 mm, but larger and smaller dimensions than these are also possible. The shape is most usually rectangular (i.e., a stone or a hone) but rounded ends are sometimes provided.
The superfinishing tools generally comprise 32%-46%, by volume, abrasive grain, 5%-20%, by volume, vitrified bond and 40-55%, by volume porosity.
The following Examples are provided by way of illustration of the invention, and not by way of limitation.